Pipeline pigs are extremely helpful in the operation of pipelines. Ordinarily, pipelines extend cross country, typically placed on the surface, but sometimes they extend under bodies of water. They can cross rivers and lakes. It's not uncommon for pipelines to pass under several bodies of water. In offshore circumstances, pipelines extend from production platforms perhaps 50 to 100 miles out in the Gulf of Mexico. Typically, the water is relatively shallow. In many places, the water is only about 100 feet deep even when the production platform is many miles offshore. In those instances, the platforms are fixed structures supported on the bottom. In deeper waters, platform erection costs become prohibitive. While two or three large platforms have been erected or perhaps have proceeded to the planning stage, it appears that the maximum limit for stationary construction of a platform is about 1,200 feet of water. Such platforms are extraordinarily expensive in that they require the frame structure as large as any of the tall buildings in the U.S. They are erected on shore and towed to an offshore location. They are deployed only when the production platform is known to be above a very prolific and highly productive field. A better approach now is the use of a tension leg platform. A tension leg platform involves a platform which is made with adequate buoyancy that it will float and hold the working deck properly above the wave action. It is held in place by cables which extend to the bottom and are anchored at the bottom. The buoyant body is held on flexible lines which are maintained in tension. It resembles a buoy which is anchored by multiple anchor lines extending to the bottom. This reduces markedly the amount of structural steel and the assembly required to assemble that structure into a supportive rigid framework. The tension legs are deployed around the platform and are anchored at the bottom.
The typical platform installed at offshore locations supports a complex amount of equipment on the deck for actual production. Indeed, the production platform may also temporarily support a drilling rig while the last of several wells are drilled. Ordinarily, a platform will not be installed unless several wells are serviced from the platform. While the number can be varied, it is not uncommon to terminate 30 to 50 wells serviced from a single platform. Many of the wells are deviated so that they actually reach bottom depth at desired locations scattered over a producing formation so that optimum production can be obtained. All the wells are therefore connected to the platform and are operated from the platform. Wellhead equipment (such as the Christmas tree) and surface gathering lines associated with land wells are not always placed on the production platform. It is desirable that wellhead equipment be located at the mudline. Typically, the wells incorporate conductor pipes to a mat or template which supports the casing and pipe strings. The many wells are terminated at the wellhead equipment located underwater. Producing wells are connected with a manifold to deliver the produced fluid and that in turn is directed into a flow line from the underwater template toward shore.
To make the example specific, assume that 40 completed wells terminate at the underwater template. Each well is provided with appropriate wellhead equipment. Wellhead valves direct their flow into a gathering line. Assume that the gathering line is a 16 inch line located in 1,000 feet of water. The gathering line delivers, under pressure, the produced oil which is pumped into the gathering line and flows to a shore location where the line emerges from the ocean. Assume that the gathering line is 200 miles long. The 200 miles of gathering line is exposed to cumulative collection of materials from the flowing oil. The materials in the gathering line often will coat the inside of the gathering line. Pipe coating is primarily dependent on the nature of the production and the temperature of the production fluids. If, for instance, the production fluids are extremely hot when produced, they will be cooled significantly in transit along the 200 mile gathering line. As cooling occurs, heavier molecules tend to coat on the pipe. It is possible that the inside diameter of the pipe will be markedly reduced by the cooling of the produced well fluids.
Sometimes, a producing well will produce small or great quantities of water which is laden with insoluble constituents. Again, the nature of the constituents will vary widely. It is however possible that they also will go out of solution and thereby plate the inside wall of the pipe. It is not uncommon for the gathering line to require cleaning periodically. Cleaning of the gathering line requires insertion of a pipeline pig into the gathering line. The pig is forced by the fluid flow from the underwater wellhead to the far end of the gathering line where it emerges onshore. In this particular instance, the gathering line must be pigged so that optimum production can be continued. If pigging is neglected, it is conceivable that the gathering line will become smaller and smaller in effective flow volume. Ultimately, serious problems can arise when the gathering line is severely plugged. It is not uncommon for the hardened materials which collect against the wall to reduce effective cross-sectional flow area by 50%. Because of that, pigging is absolutely essential to clean the line. A pig launcher is ordinarily used for lines onshore. In this instance, the pig launcher is not normally installed underwater. A vertical riser pipe is used for the pig launcher. The vertical riser must stand taller than the depth of water in which the wells are located. If the wellhead equipment is located at a depth of 1,000 feet, then the riser pipe must be 1,000 feet in height, and must have sufficient additional height so that the open end will not take any water. It is necessary to extend the pipe about 60 feet taller than the normal depth of the ocean. This vertical riser is 1,060 feet in height. This vertical gathering line riser is unwieldy. It must be stabilized laterally. Otherwise, ocean currents will force it to the side. It is sufficiently unstable in light of its relative height that it would otherwise topple. It is, however, tied to the platform above. This will add stability at the upper end so that the riser pipe will not fall over or be deflected by ocean currents or wave action.
With jack-up drilling rigs, no problem arises because the weight of the drilling rig is supported on the ocean bottom. With fixed platforms, weight does not pose a problem either because the weight is supported on the legs. With a tension leg platform, weight becomes a problem if it is excessive. In this particular instance, the gathering line riser poses a problem because it is heavy and it connects with the platform at one edge.
Again, the offset point of connection would not be a problem with a fixed platform. Here, the point of connection becomes a problem because that amount of weight at one side tends to tilt the platform because it is a floating platform. Because the platform is floating, both the weight and offset connection of the riser becomes a serious problem.
One way to overcome this is to reduce the diameter of the riser. Another weight reduction is to reduce the wall thickness. Wall thickness, however, is mandated by the depth of the water. If production is large, then a larger diameter gathering line is serviced by a larger diameter riser.
The present disclosure sets forth an improved system for gathering line pigging where the pig must be inserted through a vertical riser of several hundred feet in height. It sets forth a pig construction which is passed downwardly through the riser and then finally into the gathering line. If the vertical riser is reduced in diameter and wall thickness, a substantial reduction in weight occurs. In turn, a reduction in diameter of the riser requires a different pig construction. Therefore, another aspect of the present disclosure sets forth a different type pig. In particular, this pig is constructed to work with a bell receptacle or nipple at the top end of the riser. The pig is dropped into the riser, and lands in the bell. By gravity, it is caused to fall into the riser. Ultimately, it turns to the horizontal into the gathering line. The pig is used time and again until it is finally worn out.
The present disclosure thus describes both a riser pipe construction and a pig construction so that the pipe is made as light as possible, the pig is readily inserted, and yet the pig is permitted to shrink and then expand for actual transit through the producing line.